La végétation forestière de la Kroumirie

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Title: La végétation forestière de la Kroumirie
Author: Debazac, Eugène-Francis
Abstract: The present thesis deals with the study of the mechanical propertiesof pit props relative to various factors of forestry production.Its purposes are as follows :t . to show how wood used as supporting membres in a minebehaves in static bending and in sag, also to give some figuresfor the strength wood opposes to these stresses.2. to find how these mechanical properties will vary (as wellas warning signs given by wood before failure, and the aspectof the wood fiber where the supporting member has failed)relative to the main factors of forestry production : thespecies, the location of the tree and the type of managementadopted.3. to draw the relation that exists between mechanical strength,warning signs and the aspect of wood fibers at rupture onone hand and between certain aspects or properties of woodsuch as density, width of growth rings, moisture content, dimensionsand differences in shape, structure or chemical compositionon the other hand.In spite of considerable progress achieved these last few yearsby metal props, wood retains an important place in the mine whendifficult working conditions, the type of exploitation or kinds ofstresses call for this material. Wooden props consumption seemsto reach a limit between 20 to 25 dm3 per ton of coal produced.These figures, considering the increasing coal production willalways result in a large use of wood.But mainly when one makes a comparison with supporting membersor metal frames it is likely that users will be more and moreparticular about the quality of wood props, which is quite 'fair andwhich one should wish. Mine props will be chosen from better selectedwood. Therefore, it is important to have a good knowledge ofwood mechanical properties to investigate the factors that influencethese mechanical properties and to work out new forestry managmentsthat will produce a better raw material in the best conditions About fourty different species have been investigated, mainlyindigenous species from France but also foreign species whichhave been cultivated in our country for a various length of time.Furthermore, two bunches of mine props from Finland (spruceand Scotch Pine) and a bunch of mangrove from French Guineahave been tested for comparison.Among the fourty species studied there were 16 coniferous and24 deciduous species. In each of the two categories one single species(Scotch pine for the softwoods and oak for the hardwoods)was chosen as a reference species which was studied in details andthen compared with other species.As most of the species tested here grow in many parts of thecountry under various conditions, depending of the station andof the type of management adopted it has been necessary to considervarious test specimens for each species which were chosenaccording to special conditions so that the number of test specimensexceeds 350. For certain important species, the number of testbunches was particularly high (42 bunches of Scotch pine, 30bunches of Maritime pine (Pinus Pinaster).Finally, in order to draw valuable means and to study within abunch of specimens the interaction of certain variables such asstorage, or the influence of defects, etc... it has been necessary toprepare bunches of specimens that include a great many pieces.Sampling amounted to an average of 25 to Ioo pieces. In all, over15 000 pieces have been experimented upon.Testing ProceduresTests have been performed in compression along the grain andin bending. The ratio between length and diameter generally exceededio, compression tests were actually sag tests. Specimens testedin bending laid freely on two supports and the load was appliedat center by means of a rounded-edge knife.The testing machine consisted in hydraulic or mechanical presseswith a measuring device of the scale type, including a dynamometerof manometer and a measure of the corresponding deflectionor crush. The load at rupture was either registered during thetest on drum register or plootted afterward for each result obtained,each one corresponding to a'point on the curve.Presses used enabled us to perform tests on specimens as longas 2.5o meters under a pressure exceeding one hundred tons. Individual diagrams give the following information:r . the speed at wich the load was applied before rupture,2. a general aspect of the elastic portion which takes place atthe beginning of the test,3. the value of the load applied on the test specimen (warningload) at the precise time when it begins to give the warningsigns (either by sight or hearing),4. the maximum load and corresponding deflection,5. the aspect of the rupture,6. the work at failure which can be divided into various fractionsas indicated on graph No. i& by parallels to the « y »axes. The portion of the curve called of actual safety whichcorresponds to the breaking work in bending between the firstwarning sign (by sight) and the point where we come closerto maximum load is particularly important and must be takeninto consideration. The portion called nerve safety which laybeyond a parallel to the « y » axes corresponding to maximumload is also important and must be translated into figures.During the tests warning signs before failure, either by sight(deflection) or hearing (cracks) were carefully noted.The aspect of the rupture, the moisture content at the time ofthe test, the density determination, the growth ring counting (age)are also :among information which were written down for eachtest specimen.Presentation and Judging of ResultsResults were presented as said above, either as curves or asmomentary loads with the corresponding deflections.These figures enables us to draw or calculate the following results:I. aspect of the average curve at rupture for a given bunch,at a given moisture content,2. a comparison of curves within a given bunch, arranged accordingto the decreasing diameters or according to the decreasingmoisture contents,3. the average load for each bunch or part of a bunch of specimensand the girth corresponding to a maximum averageload per square centimeter (cm2),4. in bending, the average load corresponding to the first warningsign by sight (warning load) with the girth correspondingto the average warning load per square centimeter (cm2),5. for important bunches we have drawn a distribution diagramfor maximum load per cm2 with the total result figures obtainedin order to study the position variables and dispersion variables (see Table I, graph. 103 and 109). The study ofthese variables has enabled in many cases to see how the observeddistribution comes close to a normal distribution andto figure the percentage of results included within a scaleof given values, either near the average or above a givenfigure,6. the factor analysis within an certain number of results alsoenabled us to show clearly the influence of certain occasionalvariation factors such as accidental variation in shape, orstructure, abnormal humidity, stain or dote, etc...7. Finally, for most of the bunches tested studies of static interdependancehave been performed which result in correlativegraphs.The main correlativeness drawn was between diameter andmaximum load, but we also studied some cases of correlativenessbetween density, moisture content, soundness in relationto maximum load per cm2.Commentaries on ResultsIt is not possible of course, in that quick presentation, to giveall our results. We will limit ourselves to average results for themain species with some commentaries about the influence of thecountries where the timber has grown.a) coniferous speciesAmong coniferous species the major part has been given to fourhome grown species : Scots pine, spruce, fir and maritime pine.The sag test give comparative results for spruce and fir, 200 kgper square centimeter for pieces measuring between 45 and 70 cmin girth at their middle and 2,50 m long. Scots pine gives inferiorresults 175 kg, and maritime pine (because of its wide growthrings) is definetely weaker,129 kg.In bending, over 2 meters span still for pieces of 40 to 70 cmin girth results obtained for the above four species are nearlyidentical ranging between 18 and 18,6 kg per cm2.But, on the other hand, warning signs by sight hearing, givenbefore failure by Scots pine and maritime pine are much moreimportant than those given by either fir or spruce and their ruptureis not so fast.It is important to notice that the scattering of results is lesspronounced for species such as fir (which has not been cultivatedextensively outside its natural area in France) than for spruce(frequently introduced on lower grounds), Scots pine (for whichthere are very poor examples) or maritime pine (which often showvery wide growth rings). The well-known relation between narrow rings, high percentageof summer wood, density and higher mechanical properties has beenconfirmed and put into figures during tests performed on mineprops.With Scots pine, for example, when the width of growth ringschanges from 3 to I mm, density at 15 % rises from 0,55 to0,70 and the average maximum load per crn2 in the sag test risesfrom 125 to 225 kg (see table 8).With spruce resistances measured according to origins vary between170 and 220 kg and with fir between 152 and 227. Withmaritime pine, there is a wide range : 85 kg per cm2 for the worstand 185 kg for the best (Forest of Dom de Bormes).Everything that hampers the growth of coniferous trees withouthaving any action upon the regularity of growth rings or the physiologicstate of the tree improve the mechanical resistance of thewood. The selection of pines growing on mountains with narrowand even rings, the practise of dense forest, the growth of treeson spoor soil (but properly watered) with not too warm a station,are favorable factors for a harvest of wood with high mechanicalresistances.To these well-known coniferous species we were able to cornpareother species that are not so frequently used but among whichmany give excellent results.In sag, for example, the following loads have been registered :European larch 194 kg per cm2Cedar (from North Africa) 193 kgCorsican pine T82 kgDouglas 181 kgThese species compare favorably the species taken as standard.We shall notice that larch that comes from some altitudewith narrow rings gave a top figure of 243 kg per cm2.Next to the above species are those with relatively rapid growthwhich can he placed between Scots pine and maritime pine:Japanese larch 166 kgAustrian pine 154 kgPinus uncinata 151 kgLaricio from Calabria 142 kgThuya (western red cedar) 151 kgA coniferous species with wide rings such as Sitka spruce isclassified lower with only r24 kg.The same classification is obtained in bending:Corsican pine 26,1 kg; Douglas 22,6 ; European larch 20,9; Japaneselarch 20,6; Austrian pine 20,4; Cedar 20,3; Thuya 18,9, arecomparable to the four species studied in this report. Pinus uncinata 17,5, Laricio from 'Calabrica (with wide rings)17,4 and Sitka spruce 14,9 give lower figures.One must note that some species such as European larch, Douglas,Cedar, Corsican pine give very distinct warning signs beforerupture (cracking in sag and deflection) and their rupture is notabrupt. The species are durable and as a whcle have a perfect shape.Such species can actually be « cultivated » to give mine props.b) deciduous speciesIn the sag test some hardwoods give results similar to those obtainedwith softwoods but the majority remains inferior to softwoods.In bending though, many hardwoods show their superiority oversoftwoods. Here are the results in sag :hornbeam (sound and dry) 187 kgred oak 172 kgcommon oak 169 kg— These species give results fairly close to chose obtained withScots pine.chestnut. 144 kgalder (sound and ,dry) 142 kgare comparable in that respect to maritime pine.On the contrary,birch, aspen, lime and even robinia with 120 kg about are inferior.In bending, the results obtained with hardwoods are better :robinia (Robinia pseudoacacia) 32,5 kgash 29,5 kgred oak 27,6 kgwild cherry 22,6 kgcommon oak 22,4 kglime 20,3 kghornbeam (sound) 20,2 kgchestnut rp.8 kgfar behind come :birch 15,8 kgaspen 1 5,3 kgalder 154 kgAs a matter of curiosity let us call mangrove from Guinea whichresisted 300 kg per cm2 in sag and 48,5 kg in bending, but ofcourse, this is a record.Hardwoods give more warning signs by sight before rupture (inbending as well as in sag). Robinia, ash, oak also give a crackingsound (less pronounced though than with pines). Some hardwoods (chestnut, oak, robinia, ash, sound hornbeam)show a slow rupture of their fibers, others have an abrupt rupture(beech, even sound, maple, sorbus aria). Finally, and this is particularlyimportant, some hardwood are not durable and easely attackedby fungi (beech, hornbeam, birch, alder, aspen, Ailanthus)and because of this, lose both: their mechanical properties and theirwarning signs or security advantage.A good many hardwoods could give excellent mine props, buttheir preparation must he very carefully clone : felling when thesap is no more in circulation, grading of the pieces which must bestraight, grooving, storing in well ventilated piles to facilitate airdrying and avoid decay.c) factors (other than species and station of growth) Tc'hich have aninfuence upon mechanical properties.We have tried to evaluate the in fl uence of some factors such as:length (or span)girth (or diameter)straightnessshape of the piece (conicity)egg-like sectioncrooked graincrown of knotsmoisture contentstainetc...1) length or spanIn bending or sag_ , there is generally an inverse relation betweenspan or length and the resistance. which corroborates the generalrules of resistance of materials.One must note, nevertheless, that axial compression test wereactually sag test and not straight compression test along the grain.The relation between length and diameter was alwa.)' over 6.2) diameterIn sag we could not draw any direct relation between load persquare centimeter and section. Among the biggest pieces tested(45 to 70,cm in girth) this relation does not seem to exist (whichis logical as long as one does not change over from sag to compressionalong the grain).Nonetheless, we could notice that the load per square centimetersupported by small pieces (from 18 to 34 cm in girth) was the sameas the load supported by bigger pieces when the former were testedin sag on 1,60 ni only instead of 2,50 ni in length. It wouldthen seem interesting to resume these surveys of relationship basedon the ratio: length over diameter of tested specimens, In bending, we found a relation though not very conspicuous,between the mechanical resistance and thediameter within thehunch of specimens tested over 2 ni span (45 to 70 cm in girth).This relation is not paramount because the girth often brings otherweakness factors such as large knots, greater moisture content,stain, heart checks, etc...The relations studied have enabled us to figure out a shape factorwhich is the « n ]» of the well-known formula of « fatigue »given by the AFNOR standards.3 PLFu —2hh°in whichFt' is the value of fatigue in bendingP the load at failureL the spanb and h the horizontal and vertical dimensions of the sectionn the shape factor.We found for mine props a shape factor slightly lower thanthe one given by Marcel Monnin for structural lumber n 6/5instead of 8/6.We have also noticed that smaller pieces (from 18 to 34 cm ingirth) showed a resistance in bending per cm2 over a one-meterspan identical to the resistance in bending over a two-meter spanshown by large pieces. During the time spent for this survey wecould not study the influence of girth for one single span overa large range of girth (from T8 to 70 cm), but a complementarystudy could he made on that particular subject.3) straightnessWithout a detailed study we shall note that one single bend. inferiorto 3 cm per meter of length will result in a TO %^ to 20reduction in the resistance in sag.In the case of a double bend irg S. the reduction will reach 30 %.An exagerated conicity (2 to '2.5 cm per meter, over the diameter)will result in a TS to 20 % reduction in the sag test.If the tree shows too much of an egg shape (1 s 2/3 L) thiswill also reduce the resistance in bending by 20 °Jo.4) defectsCrown of conspicuous knots of small sizes does not seem tohave any action either in bending or in sag. On the contrary,when knots are over 2 cm in diameter the resistance in bending isreduced, failure « jumps » toward the crown of knots and becomesabrupt. One notices a reduction in the maximum load ranging from 72 to i8 %. In sag the reduction of resistance appears whenknots are 3 cm in diameter or over. This reduction reaches io to15 %. Props then crak at the knots whether they are conspicuousor slightly covered. Early artificial pruning which will result in alarger percentage of defectless timber will at the same time increasethe resistance of props.5) moisture contentMoisture content has a definite influence on the resistance ofwood. Standards published by AFNOR give formulae to changethe resistance of a given piece of wood (measured at H% MC) toan average MC (r5 %).For example, the stress at rupture in bending:F15'= FH(r -f- c' (H 1 5))in whichc' is a factor called « humidity factor ».We were able to figure out these factors in sag and in bendingfor some species. We found for example, in sag:2 to 3 % for Douglas fir and mountain larch3 to 4 % for 'fir, spruce and Scots pine4 to 5 % for larch growing in plains and hardwoods (oak, chestnut,hornbeam)5 to 6 % for Austrian black pine and maritime pine with widerings.In bending we found :2 % for Douglas fir, white fir, spruce, larch2 to 3 % for oak, chestnut, hornbeam, Scots pine3 to 4 % for maritime pine and Austrian black pine.This is very important to judge the resistance of a given pieceof timber according to its moisture content. Moisture content variesnot only after felling, to reach the equilibrium with the relativehumidity and the temperature of the storage place but alsoshows a certain up and down variation with an increase in moistureduring cold, wet seasons.6) decaySoftwoods, when they are barked as soon as felled (which isparticularly important in the case of Austrian Black pine) areless exposed to stain. Nevertheless, large pine logs often showstain signs which are not very dangerous in themselves but mayhide some worse stain inside the tree. On low grounds one mustavoid felling pines when sap is circulating (burnt trees show thesame risks).Decayed wood is easily recognized because it smells of etherand because of a certain hysteresis over summer drying (decayed wood gains more water during wet seasons and loses less duringdry seasons).Many hardwoods (deciduous species) oak, chestnut, locust, etc...are durable, others such as Ailanthus, alder, birch, hornbeam, beech,aspen, etc... are likely to get decayed and sometimes in a veryhad way. If felled when the sap is in motion they get stained atonce. If, on the contrary, they are felled after the sap has stop -ped circulating inside the tree some of them (birch, hornbeam)can last one year but all get stained during the second year. Inthat case, the resistance is reduced by half it not more, and ruptureis always abrupt (brash) without warning signs.The preparation and use of birch and hornbeam as mine propsmust be carried out with that serious disadvantage in mind.
Publisher: ENEF, Ecole nationale des eaux et forêts, Nancy (FRA)
Date: 1959

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